1. Field of the invention
This invention relates to a display apparatus, and more particularly, to a color display apparatus where a display image formed by an electro-optical cell is enlarged and projected onto a screen.
2. Description of the prior art
FIG. 5 shows the structure of a conventional liquid-crystal color display cell. This structure is the same whether the liquid-crystal display cell is driven by an active matrix drive or a multiplex drive. The substrate 101 on one side is provided with color picture elements 500a, 500b, 500c, . . . , which are in the form of a high-accuracy mosaic pattern of red, green, and blue colors. On the surface of the color picture elements, there is a transparent electrode 201 made of a transparent conductive material such as indium tin oxide (ITO) or the like. On the surface of 201, there is formed a molecular alignment film 301 that is made of polyimide or the like. The molecular alignment film 301 having been treated by an aligning treatment such as rubbing, etc. The liquid-crystal layer 400 is in contact with this molecular alignment film 301. On the surface of the other substrate 100, there are formed transparent electrodes 200a, 200b, 200c, . . . , and on their tops, there is a molecular alignment film 300 that has been treated by an aligning treatment as described above. The liquid-crystal layer 400 is in contact with this molecular alignment film 300.
In general, the directions of the molecular alignment on the two substrates 100 and 101 can be rotated through an angle of 90.degree. to each other. The liquid-crystal cell mentioned above is inserted between two polarizing plates. If the polarizing axes of the two polarizing plates are parallel to each other and the liquid crystal is the nematic one with positive dielectric anisotropy, the portions of the cell to which a voltage is not applied do not transmit light. For example, in FIG. 5, when the voltage above the threshold voltage is applied to the space between the electrode 200b and the transparent electrode 201 only green light can pass through the corresponding portion of the cell, and the display will be green. In this way, the color display image is produced by controlling the transmittance of the light that can pass through the color picture elements 500a, 500b, 500c, . . . of red, green, and blue colors.
In the liquid-crystal color display cell described above, the color picture elements 500a, 500b, 500c, . . . , are in the form of a mosaic pattern and are integrated with the substrate 101, so that when these color picture elements 500a, 500b, 500c, . . . , are formed on the substrate 101, with approximately the same accuracy in production as is needed as for the picture element electrodes 200a, 200b, 200c, . . . Usually, a color filter is produced as follows: by photolithography, a filter substrate made of gelatin or the like is coated with a resist layer that has openings only in the desired regions, and the coloring step with a pigment is repeated at least three times. Because the process of manufacture is so complicated, the production cost of the color filter is extremely high. If the resistance to heat and to chemicals and similar properties of the matrix such as gelatin and the like are considered, the serious limitations to each step of washing, formation of the transparent electrodes, and formation of the molecular alignment film are apparent.
The production yield of the final liquid-crystal display cell is evaluated as the product of the yield of the substrate with a color filter and the yield at the time of the attachment to the other substrate, so it is difficult to attain a high production yield.
A liquid-crystal display cell of this type is used for many applications, such as a projection-type color display apparatus shown in FIG. 6. In this color display apparatus, the liquid-crystal display cell 44 is illuminated with light by means of the reflecting mirror 14, light source 24, and condenser lens 34, and the color display image produced on the liquid-crystal display cell 44 is enlarged and projected onto the screen 76 via the projection lens 54. In order to realize sufficiently high brightness of the display image on the screen, the intensity of light which enters the liquid-crystal display cell 44 must be very high, which causes fading of the color filter.